Zombies Ate My Nieghbors
I seriously probably died and had to repeat the first few levels so many times that, when I booted it up on an emulator a few months ago, I remembered pretty much everything about the first five levels.
You take the place of one of two teens (or both, in multiplayer) who are all that stand between the teeming hordes of zombies, werewolves, and b-movie monsters, and, well, your hapless neighbors.
Scattered around each level are a few of your cowering co-humans who've got to rescue before they get eaten up. Saving more earns you more points and a few extra lives, which can come quite in handy as this game is pretty hard.
For bosses, you'll fight giant babies and possessed toys and all sorts of weirdness.
This game was actually re-released and slightly remastered for the Wii a little while ago, so if you have one it would be worth checking out, especially since it's co-op.
Because of it's popularity, it's also pretty easy to find a decent ROM of it.